Lookup fields and formula fields DO NOT lend themselves to be modified by any thing other than what it is intended by. For ex. a formula field is calculated by the formula. You can't modify its value. You can however, modify the fields that contribute to this formula field. Those are the source. Likewise, a lookup field comes from its parent table. If it has to be modified, you would change its value on that table. That is the source.

I am trying to do an initial upload of my product catalog. For example, I need to look up in the HTS table to find the duty rate & HTS# of the product. I want to be able to import the values from a csv because I have over 500 styles.

If you have a table of duty rates in a table called HTS, you would change the Key field of that table to be the HTS #. But in fact if you are importing from more than one country you would need to create a Key field which is a concatenation of the HTS and the country.

such as

0901.21.0050-Canada

The HTS table has the duty rate.

The on your styles table you would create that same formula to result like

0901.21.0050-Canada

using a formula text field

Then make a relationship based on that concatenated field and lookup your duty rate.

The first is to have a formula to calculate the key field and then a form rule to copy that to the real Key field.

The second is that in fact you can have a formula field be a Key field by typing in the API. I have found that works fine, except for importing such as imports off excel or table to table saved imports. it one of those undocumented things that i learned at an EMPOWER one year in a Q & A session.

Do you have duty rates by country? Do you get what i mean by the concatenated field? and see my post above with using a form rule to enter the formula value into a field that will be the Key field to the HTS table.